HOw old are the kids? I went as a teen and then subsequently as an adult so I am not very au fait with stuff for younger kids.
The Pergammon museum is stunning - I remember being over-awed even as a shallow teen.
I think that the House at CHeckpoint CHarlie is still there - it was a fab museum, very cluttered but infinitely fascinating. It may or may not have been poshed up.
The Jewish Museum and Memorial are unmissable, imo but tough.
The only castle is really the Charlottenburg but its more a palace than a castley-castle. For those you need to journey down the Rhine.
The Reichstag dome is a very popular tourist thing. As a kid, I remember liking walking round the Food Hall in the KDW (Kaufhaus Des Westens - posh dept store).
I'd be tempted to do a cycle tour - its a flat city so easy to get around and some the big spaces are a long walk. There is also quite a lot of interesting modern architecture so getting around on a bike is a good way to see it all.
DH & I watched some midnight cabaret but prob not for kids!
Its an amazingly vibrant place - I love it. I am happy just wandering around finding markets and beer gardens and mad art galleries. I hope you have a fab time.